FreeBSD Man Pages

SENDFILE(2) FreeBSD System Calls Manual SENDFILE(2)
NAMEsendfile -- send a file to a socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h>
intsendfile(intfd, ints, off_toffset, size_tnbytes,
structsf_hdtr*hdtr, off_t*sbytes, intflags);
DESCRIPTION
The sendfile() system call sends a regular file specified by descriptor
fd out a stream socket specified by descriptor s.
The offset argument specifies where to begin in the file. Should offset
fall beyond the end of file, the system will return success and report 0
bytes sent as described below. The nbytes argument specifies how many
bytes of the file should be sent, with 0 having the special meaning of
send until the end of file has been reached.
An optional header and/or trailer can be sent before and after the file
data by specifying a pointer to a structsf_hdtr, which has the following
structure:
struct sf_hdtr {
struct iovec *headers; /* pointer to header iovecs */
int hdr_cnt; /* number of header iovecs */
struct iovec *trailers; /* pointer to trailer iovecs */
int trl_cnt; /* number of trailer iovecs */
};
The headers and trailers pointers, if non-NULL, point to arrays of structiovec structures. See the writev() system call for information on the
iovec structure. The number of iovecs in these arrays is specified by
hdr_cnt and trl_cnt.
If non-NULL, the system will write the total number of bytes sent on the
socket to the variable pointed to by sbytes.
The flags argument is a bitmap of these values:
SF_NODISKIO. This flag causes any sendfile() call which would
block on disk I/O to instead return EBUSY. Busy servers may bene-
fit by transferring requests that would block to a separate I/O
worker thread.
SF_MNOWAIT. Do not wait for some kernel resource to become avail-
able, in particular, mbuf and sf_buf. The flag does not make the
sendfile() syscall truly non-blocking, since other resources are
still allocated in a blocking fashion.
SF_SYNC. sendfile sleeps until the network stack no longer refer-
ences the VM pages of the file, making subsequent modifications to
it safe. Please note that this is not a guarantee that the data
has actually been sent.
When using a socket marked for non-blocking I/O, sendfile() may send
fewer bytes than requested. In this case, the number of bytes success-
fully written is returned in *sbytes (if specified), and the error EAGAIN
is returned.
IMPLEMENTATION NOTES
The FreeBSD implementation of sendfile() is "zero-copy", meaning that it
has been optimized so that copying of the file data is avoided.
TUNING
On some architectures, this system call internally uses a special
sendfile() buffer (structsf_buf) to handle sending file data to the
client. If the sending socket is blocking, and there are not enough
sendfile() buffers available, sendfile() will block and report a state of
``sfbufa''. If the sending socket is non-blocking and there are not
enough sendfile() buffers available, the call will block and wait for the
necessary buffers to become available before finishing the call.
The number of sf_buf's allocated should be proportional to the number of
nmbclusters used to send data to a client via sendfile(). Tune accord-
ingly to avoid blocking! Busy installations that make extensive use of
sendfile() may want to increase these values to be inline with their
kern.ipc.nmbclusters (see tuning(7) for details).
The number of sendfile() buffers available is determined at boot time by
either the kern.ipc.nsfbufsloader.conf(5) variable or the NSFBUFS kernel
configuration tunable. The number of sendfile() buffers scales with
kern.maxusers. The kern.ipc.nsfbufsused and kern.ipc.nsfbufspeak read-
only sysctl(8) variables show current and peak sendfile() buffers usage
respectively. These values may also be viewed through netstat-m.
If a value of zero is reported for kern.ipc.nsfbufs, your architecture
does not need to use sendfile() buffers because their task can be effi-
ciently performed by the generic virtual memory structures.
RETURN VALUES
The sendfile() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
[EAGAIN] The socket is marked for non-blocking I/O and not all
data was sent due to the socket buffer being filled.
If specified, the number of bytes successfully sent
will be returned in *sbytes.
[EBADF] The fd argument is not a valid file descriptor.
[EBADF] The s argument is not a valid socket descriptor.
[EBUSY] Completing the entire transfer would have required
disk I/O, so it was aborted. Partial data may have
been sent. (This error can only occur when
SF_NODISKIO is specified.)
[EFAULT] An invalid address was specified for an argument.
[EINTR] A signal interrupted sendfile() before it could be
completed. If specified, the number of bytes success-
fully sent will be returned in *sbytes.
[EINVAL] The fd argument is not a regular file.
[EINVAL] The s argument is not a SOCK_STREAM type socket.
[EINVAL] The offset argument is negative.
[EIO] An error occurred while reading from fd.
[ENOTCONN] The s argument points to an unconnected socket.
[ENOTSOCK] The s argument is not a socket.
[EOPNOTSUPP] The file system for descriptor fd does not support
sendfile().
[EPIPE] The socket peer has closed the connection.
SEE ALSOnetstat(1), open(2), send(2), socket(2), writev(2), tuning(7)
K. Elmeleegy, A. Chanda, A. L. Cox, and W. Zwaenepoel, "A Portable Kernel
Abstraction for Low-Overhead Ephemeral Mapping Management", TheProceedingsofthe2005USENIXAnnualTechnicalConference, pp 223-236,
2005.
HISTORY
The sendfile() system call first appeared in FreeBSD 3.0. This manual
page first appeared in FreeBSD 3.1.
AUTHORS
The sendfile() system call and this manual page were written by David G.
Lawrence <dg@dglawrence.com>.
FreeBSD 10.1 January 7, 2010 FreeBSD 10.1